Axial-piston machine having an antiwear layer

ABSTRACT

An axial piston machine having a control plate and a cylinder drum rotating relative to the control plate. The control plate rubs or slides on a second sliding side of the cylinder drum via a first sliding side. One of the sliding sides has a carbon-containing layer.

FIELD OF THE INVENTION

The present invention relates to a hydrostatic machine configured asaxial-piston machine.

BACKGROUND INFORMATION

An axial-piston machine having a control plate and a cylinder drumsliding on the control plate is described in German Patent ApplicationSerial No. DE 102 23 844 A1, for instance. Situated on one side of thepaired surfaces rubbing against each other is a plastic layer, and onthe other side is a carbon-containing layer.

Due to the two-axle loading in the axial and radial direction, andbecause of cavitation, the regions of the surfaces of the control plateand the cylinder drum facing each other are subjected to increasedloading. This includes the normally present notches to avoid rapidpressure fluctuations at the control openings of the control plate. Inparticular, the surfaces facing each other are also stressed byrebounding of the cylinder drum or the pressure plate on to the controlplate.

A disadvantage of the aforementioned axial piston machine is that in thepresence of oil-containing flow media, the plastic layer may react tothe flow medium in a disadvantageous manner, which subjects the plasticlayer to increased wear. In particular in rapid load changes and at highoperating pressures, vapor bubbles can form, and subsequent pressurepulses, so-called cavitation, may occur in the hydraulic medium, as wellas the already mentioned rebounding of the cylinder drum or the pressureplate; the plastic layer has only insufficient resistance to suchstresses and thus wears quickly, the plastic layer detaching onlyregionally, which results in uneven loading and faster wear.

From the related art it is also known to fuse a bronze layer onto thesliding side of the cylinder drum.

In addition, a production method for producing a diamond-like, amorphouscarbon layer is described in International Application No. WO 98/54376A1.

Furthermore, the publication “Helena Roukainen, Tribological propertiesof hydrogenated and hydrogen-free diamond-like carbon coatings”, VTTPublications, 2001, describes the ta-C or CVD method, specifically onpages 3 and 4 and pages 26 to 28.

Both the plastic layer and the bronze layer increase the production timeand expense and are disadvantageous as a result.

SUMMARY

It is an object of the present invention to provide an axial pistonmachine that is able to be manufactured in a less complicated manner,may be operated with a multitude of flow media and is less susceptibleto wear, especially under heavy loading.

An axial piston machine according to an example embodiment of thepresent invention, may provide the advantage of higher wear resistance,in particular with respect to knocking loading of the parts slidingalong each other, with respect to cavitation and to the two-axleloading. Furthermore, the axial piston machine according to the exampleembodiment of the present invention is easier to produce and, inaddition to water, may also be operated using oil-containing flow media.

In particular, it may be advantageous if the respective other slidingside not coated by the layer is made of steel which is hardened bynitration. This allows an easier manufacture of the sliding side lyingopposite the carbon layer. The use of environmentally damagingnonferrous metals may be dispensed with.

Furthermore, it may be advantageous if the layer is applied on the firstsliding side or on the sliding side of the control plate. Since thecontrol plate of an axial piston machine has smaller dimensions than thecylinder drum, the layer is able to be applied on the control plate in asimpler and more cost-effective manner.

In addition, it may be advantageous if the control plate and/or thecylinder drum are/is generally made of metal in the region of theirrespective sliding sides, and if the layer is applied directly onto themetal surface of the sliding sides. The layer then adheres to therespective sliding side in a more durable manner. The functional layermay be applied either directly or preferably with the aid of an adhesivelayer.

In addition, it may be advantageous if the layer is a diamond-like,amorphous carbon layer, in particular a tetrahedral, hydrogen-freeamorphous carbon layer, ta-C. Compared to conventional carbon layers,the ta-C layer has especially advantageous characteristics when used inan axial piston machine. For instance, the fatigue resistance of thefriction- and wear-reducing layer with respect to adhesive chipping andcohesive erosion, in particular, is increased considerably, especiallywith respect to the stresses by cavitation and the impact forces thatoccur at increased operating pressure and in rapid, heavy load changes,in particular. The wear resistance is improved, especially givenincreased loading and contaminated flow media.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described below in greater detail by wayof example on the basis of an exemplary embodiment and specific exampleembodiments. Identical components have been provided with matchingreference numerals.

FIG. 1 shows a schematic representation of an axial piston machineaccording to an example embodiment of the present invention.

FIG. 2 shows cut-away portion II shown in FIG. 1, in enlarged form.

FIG. 3 shows a preferred specific embodiment of a control plate of theaxial piston machine according to the present invention, in a plan viewof the sliding slide of the control plate.

FIG. 4 shows a preferred specific embodiment of a cylinder drum of theaxial piston machine according to the present invention, in a plan viewof the sliding side of the cylinder drum.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The axial piston machine illustrated in FIG. 1 is configured as swashplate with an adjustable displacement volume and one flow direction; inthe conventional manner it includes a hollow-cylindrical housing 1having an end that is open at the end face (upper end in FIG. 1); aconnection plate 2, which is secured to housing 1 and seals its openend; a cam plate or swash plate 3; a control plate 4, which is alsoknown as control body or control mirror; a drive shaft 5 and a cylinderdrum 6.

Swash plate 3 is designed as so-called tilting cradle having asemi-cylindrical cross section and, via two bearing surfaces that extendwith mutual clearance parallel to the tilting direction, is supportedwith hydrostatic destressing at two correspondingly formed bearingshells 8, which are mounted on the inner surface of housing end face 9situated opposite connection plate 2. The hydrostatic destressing isimplemented in a conventional manner via pressure pockets 10 formed inbearing shells 8 and supplied with pressure medium via connections 11.An actuating device 13, which is accommodated in a bulge of acylindrical housing wall 12, engages with swash plate 3 via an arm 14that extends in the direction of connection plate 2 and is utilized totilt the same about a tilting axis that is perpendicular to the tiltingdirection.

Control plate 4 is centered on the outer ring of bearing 18 and,positioned in the circumferential direction, rests against the innersurface, facing the housing interior, of connection plate 2. Controlplate 4 is provided with two straight-through openings 15 in the form ofkidney-shaped control slots, which are connected to, respectively, apressure and suction line (not shown) via a pressure channel 16D orsuction channel 16 in connection plate 2. Pressure channel 16D has asmaller flow cross section than suction channel 16S. The control surfaceof control plate 4 facing the housing interior and having a sphericaldesign is used as bearing surface for cylinder drum 6.

Drive shaft 5 projects into housing 1 through a through-hole in housingend face 9 and is rotatably supported in connection plate 2 with the aidof a bearing 17 in this through-hole, and with the aid of anotherbearing 18 it is supported in a more narrow bore section of a blind holebore 19 widened at the end face. Furthermore, in the interior of housing1, drive shaft 2 penetrates centrical through-hole bore 20 in controlplate 4, a centrical through-hole bore 21 in swash plate 3, as well as acentrical through-hole bore in cylinder drum 6 having two bore sections.

One of these bore sections is formed in a sleeve-shaped extension 23,which is premolded on cylinder drum 6, projects beyond its end face 22facing swash plate 3 and is utilized to connect cylinder drum 6 to driveshaft 5 in a torsionally fixed manner, with the aid of a wedge-grooveconnection 25. The remaining bore section has a conical design. Ittapers from its cross section having the largest diameter, close to thefirst bore section, to its cross section having the smallest diameter,close to the end or bearing surface of cylinder drum 6 resting againstcontrol plate 4. The annular space defined by drive shaft 5 and thisconical bore section is denoted by reference numeral 25.

Cylinder drum 6 has stepped cylinder bores 26, which generally extend inthe axial direction and are evenly disposed on a graduated circle thatis coaxial with respect to the axis of the drive shaft. Cylinder bores26 discharge directly at cylinder drum end face 22 and, via end channels27, at the cylinder drum bearing surface facing control plate 4, on thesame graduated circle as the control slots. One cylinder sleeve 28 ineach case is inserted in the cylinder bore sections having a largerdiameter and discharging directly at cylinder drum end face 22. Cylinderbores 26 including cylinder sleeves 28 are denoted as cylinders here.Pistons 29, disposed within these cylinders so as to be displaceable,have spherical heads 30 at their ends facing swash plate 3, which aremounted in slide shoes 31 and are hydrostatically supported on anannular sliding pad 32 mounted on swash plate 3 via these sphericalheads 30.

On its sliding surface facing sliding pad 32, each slide shoe 31 isprovided with its own pressure pocket (not shown), which is connected toa stepped axial through channel 34 in piston 29 via a through hole 33 inslide shoe 31 and is thereby connected to the working chamber of thecylinder delimited from piston 29 in cylinder bore 26. A throttle isformed in each axial through channel 34 in the region of the assignedspherical head 30. A holding-down clamp 36, which is situated on driveshaft 5 so as to be axially displaceable, utilizing a wedge-grooveconnection 24, and which is acted upon in the direction of swash plate 3by a spring 35, retains slide shoe 31 in contact with sliding pad 32.

The function of afore-described axial piston machine 1 is generallyconventionally, which is why the following description of its use aspump is limited to the main portions.

Axial piston machine 1 is intended for an operation using oil as flowmedium. Cylinder drum 6 together with pistons 29 is made to rotate viadrive shaft 5. When an activation of actuating device 13 has broughtswash plate 3 into a tilted position relative to cylinder drum 6, allpistons 29 execute translational movements. When cylinder drum 6 isrotated about 360°, each piston 29 executes an aspiration and acompression stroke during which corresponding oil flows are generatedwhose conveyance and evacuation is implemented via end channels 27,control slots 15 and pressure and suction channel 16D, 16S,respectively. During the compression stroke of each piston 29pressurized oil flows from the individual cylinder into its pressurepocket via axial through channel 34 and through hole 33 in associatedslide shoe 31 and generates a pressure field between sliding pad 32 andrespective slide shoe 31, which is utilized as hydrostatic bearing forthe latter. Furthermore, via connections 11, pressurized oil is conveyedto pressure pockets 10 in bearing shells 8 to support swash plate 3hydrostatically.

Sliding surfaces 44, 45, shown in greater-detail in FIG. 2, are formedon the facing sides of control plate 4 and cylinder drum 6. Controlplate 4 has first sliding side 44, and cylinder drum 6 has secondsliding side 45. A friction- and wear-reducing layer 46, which is shownin greater detail in FIG. 2, has been applied on first sliding side 44,between the two sliding sides 44, 45.

FIG. 2 shows cutaway II illustrated in FIG. 1 in an enlarged view, theregion around layer 46, control plate 4, and cylinder drum 6, as well asaround first sliding side 44, second sliding side 45, and opening 15being shown in enlarged form as a result. In the exemplary embodiment,layer 46 is situated on first sliding side 44, i.e., on control plate 4.Layer 46 is preferably made of a tetrahedral, hydrogen-free amorphousdiamond-like carbon, which is known by the abbreviation ta-C, from thegroup of DLC layers (diamond like carbon layers).

Layer 46, which reduces friction and protects against wear, is evenlyapplied on first sliding side 44 by a PVD method (physical vapordeposition), for instance, or by the specialized arc-PVD method or a CVDmethod (chemical vapor deposition), preferably, however, with the aid ofa PECVD method (plasma enhanced chemical vapor deposition). Furthermore,layer 46 is formed as so-called thin layer, at a thickness of up toapproximately 15 micrometers, a range of 1 to 3 micrometer beingendeavored. A metallic adhesion layer is normally used, in particularmade of Cr, Ti, Zr.

Second sliding side 45 of cylinder drum 6, which lies opposite layer 46and is likewise made completely of steel, is hardened, preferably bynitration.

During operation of axial piston machine 1 designed according to thepresent invention by way of example, the rotational movement causes ahydrodynamic sliding film to form between layer 46 and second slidingside 45, which is made up of the flow medium. However, in startupoperation, layer 46 rubs against second sliding layer 45. Due to thehydraulic pressure fluctuations that occur in axial piston machine 1,especially when the axial piston machine is subjected to heavy loading,second sliding side 45 may briefly and rapidly drop back onto layer 46again. This causes knocking loading, whose intensity is a function ofthe working pressures and the pressure fluctuation profile, inparticular.

FIG. 3 shows a preferred specific embodiment of a control plate 4 ofaxial piston machine 1 according to the present invention, in a planview of sliding side 44 of control plate 4. Easily visible in this vieware the approximately kidney-shaped openings 15, which are used tocontrol the filling and evacuation of cylinder bores 26. In theexemplary embodiment, both openings 15 have a notch 47 at each of theirends disposed in the direction of rotation. Notches 47 cause a softreversing and are known as such from the related art. Sliding side 44 ofcontrol plate 4 is completely coated by layer 46, the surfaces ofnotches 47 and the regions of the inner sides of openings 15 beingincluded within the meaning of the present invention. In other exemplaryembodiments, in order to have the reversing occur in a softer manner,notches (not shown) may be disposed on the end situated counter to thedirection of rotation, or bores may be introduced into control plate 4;the surfaces of these notches (not shown) and the inner surfaces of thebores (not shown) would likewise be coated by layer 46 according to thepresent invention.

In axial piston machines material stresses caused by cavitation tend tooccur in a region 48, which has been outlined by a circular ring by wayof example.

FIG. 4 shows a preferred specific embodiment of a cylinder drum 6 ofaxial piston machine 1 according to the present invention, in a planview of sliding side 45 of cylinder drum 6, which is denoted as secondsliding side 45. Regions 48, which are outlined by circular rings inthis figure and mark the regions at risk by cavitation, lieapproximately between end channels 27 formed as elongated holes, theends of end channels 27 being outlined as well. According to the presentinvention, second sliding side 45 instead of first sliding side 44 maybe coated by layer 46, the inner surfaces of end channels 27 preferablyalso being coated by layer 46 in this case.

The present invention is not restricted to the exemplary embodiments andspecific embodiments. The features of the exemplary embodiment and thespecific embodiments may be combined with each other as desired.

1-8. (canceled)
 9. An axial piston machine, comprising: a control plate;and a cylinder drum which rotates relative to the control plate, thecylinder drum and the control plate rubbing or sliding against eachother, the control plate rubbing or sliding on a second sliding side ofthe cylinder drum via a first sliding side of the cylinder drum, whereinone of the first sliding side and the second sliding side has acarbon-containing layer, and the other of the first sliding side and thesecond sliding side is made of metal.
 10. The axial piston machine asrecited in claim 9, wherein the sliding side made of metal is made ofsteel hardened by nitration.
 11. The axial piston machine as recited inclaim 9, wherein the layer is applied on the first sliding side.
 12. Theaxial piston machine as recited in claim 9, wherein at least one of thecontrol plate and the cylinder drum is made of metal in a region of thefirst and second sliding sides, and the layer is applied directly on ametal surface of the one of the first or second sliding side.
 13. Theaxial piston machine as recited in claim 9, wherein the layer is adiamond-like amorphous carbon layer.
 14. The axial piston machine asrecited in claim 13, wherein the layer is a tetrahedral, hydrogen-freeamorphous carbon layer (ta-C).
 15. The axial piston machine as recitedin claim 9, wherein the layer is a diamond layer, the diamond layerbeing one of a nanocrystalline, microcrystalline or doped CVD diamondlayer.
 16. The axial piston machine as recited in claim 9, wherein oneof the first and second sliding sides spherically projects into theother sliding side.